As part of energy saving policies, there is an ever-increasing demand for lower fuel consumption of automobiles as well. In particular, there is an increasing demand from automobile manufacturers to reduce torque of bearings for supporting automotive transmissions and differentials for the purpose of achieving lower fuel consumption. As one measure to achieve lower fuel consumption, tapered roller bearings, which have been used thus far, are replaced by ball bearings having lower torque than the tapered roller bearings (type of bearing is changed).
As illustrated in FIG. 8, a general related-art ball bearing (deep-groove ball bearing) includes an outer race 2 having a circular-arc outer rolling surface 1 formed on an inner circumference thereof, an inner race 4 having a circular-arc inner rolling surface 3 formed on an outer circumference thereof so as to face the outer rolling surface 1, a retainer 5 arranged between the inner race 4 and the outer race 2, and a plurality of balls Bo rollably supported by the retainer 5.
As illustrated in FIG. 9, the retainer 5 includes two annular retaining plates 7 and 7 combined with each other. The two annular retaining plates 7 and 7 each have semispherical bulging portions 6 arranged at predetermined intervals along a circumferential direction of the retainer 5. Specifically, each of the annular retaining plates 7 includes the semispherical bulging portions 6 arranged along the circumferential direction and flat portions 8 between the semispherical bulging portions 6 adjacent to each other. In a combined state, the flat portions 8 and 8 are superimposed on each other, and the flat portions 8 and 8 are coupled to each other through intermediation of fasteners 9 such as rivets. Thus, the semispherical bulging portions 6 face each other to form ball-fitting portions (pockets) 10 each having a ring-like shape.
In recent years, to meet the demand for lower fuel consumption, further torque reduction has been desired for the ball bearing as well despite the fact that the torque is originally low. The torque reduction can be achieved through changes in internal design specifications (reduction in number of balls, reduction in contact area between the ball and a guide surface of the retainer, and the like), which may however lead to excessive contact pressure, shorter life, or other influence on the functions of the bearing. In addition, significant change in shape of the retainer may cause cost increase due to the need for a new mold. Thus, it is not preferred to change the internal design specifications of the bearing having standard specifications, and to significantly change the shape of the retainer.
In this case, the ratio of torque-generating factors in the torque of the deep-groove ball bearing is confirmed to be rolling torque (26%) generated due to contact between the ball and the guide surface (rolling surface of each of the inner and outer races), oil shear torque resistance (71%) generated due to contact between the ball and a guide surface of the pocket (ball contact portion of the retainer), and others (3%). Thus, to achieve torque reduction, it is effective that the oil shear torque generated due to the contact between the guide surface of the pocket and the ball be reduced because of its higher torque ratio.
The bearing used for the confirmation of the ratio of torque-generating factors was a bearing having an inner diameter of φ35 mm, an outer diameter of φ72 mm, and a width of 17 mm (bearing #6207 manufactured by NTN Corporation). Further, the material for the retainer in use was iron. As experimental conditions, the radial load was set to 500 N, the rotational speed was set to 4,000 r/min, the type of lubricating oil was set to automatic transmission fluid (ATF), and the lubrication conditions were set to spraying or splashing.
By the way, to reduce the oil shear torque generated due to the contact between the guide surface of the pocket (ball contact portion of the retainer) and the steel ball (ball), the contact area therebetween only needs to be reduced simply. In the related art, there are ball bearings constructed so as to reduce the contact area between the guide surface of the pocket (ball contact portion of the retainer) and the ball (Patent Literatures 1 and 2).
In Patent Literature 2, elongate holes are formed in the guide surface of the pocket (ball contact portion of the retainer), and auxiliary recesses are formed on an inner circumferential side of the pocket. The elongate holes and the recesses are formed as described above, and thus the contact area between the guide surface and the ball can be reduced.